1. Field of the Invention
The present invention relates to the field of fabricating photomasks for use in the manufacture of semiconductor devices and, more particularly, to the fabrication of phase shifting photomasks for use in submicron lithography techniques.
2. Related Application
This application is related to copending application entitled "Globally Planarized Binary Optical Mask Using Buried Absorbers," Ser. No. 08/342,940, filed Nov. 21, 1994, which is incorporated by reference herein.
3. Prior Art
As semiconductor technology evolves to allow for ever smaller device structures to be fabricated on a wafer, such as a silicon wafer, photomask technology must also improve in order to pattern ever-smaller features on the semiconductor wafer. It is the patterns present on these photomasks that are imaged onto the wafer to define the various features formed on the wafer.
Due to the limitations imposed by the wavelength of light, patterns imaged onto the wafer tend to degrade as feature size shrinks. Various schemes have been devised to extend the lower range limitations imposed by conventional optical lithography. One technique has been the use of ultra-violet light in what is currently known as Deep Ultra-Violet (DUV) to utilize the shorter wavelength in the range of 248 and 193 nanometers. Other more radical techniques suggested are electron-beam (e-beam) and x-ray usage to pattern images at lower submicron ranges. However, recent experimentation in the area of phase shifting masks (PSMs) have shown that PSM technology can be employed to extend the range of optical techniques currently being employed. With phase shifting at DUV wavelengths, optical lithography can be extended to the lower submicron ranges previously considered unattainable by optical lithography.
Phase shifting techniques have been generally known as early as U.S. Pat. No. 4,360,586. One type of PSM that has shown considerable promise is the attenuated phase shifting mask in which an attenuator attenuates the transmitted light passing through it, at the same time shifting the phase of the light (typically, by 180.degree.). One such attenuated PSM is described in U.S. Pat. No. 4,890,309. Attenuated PSMs have emerged as one of the preferred, if not the most preferred, practical approaches to enhancing the lithography "process latitude" or "process window" (depth of focus.times.exposure latitude) over conventional binary intensity masks.
The attenuator is usually formed from a metallic like light absorbing film, such as chrome or chrome oxide, that has been thinned to allow for about 5%-15% of the incoming light to be transmitted through the material. This partial transmittance of the optical beam through the phase attenuator allows phase shifted light to be produced, which can "interfere" with the non-phase shifted light, thereby improving the edge sharpness and increasing the depth of focus. The thickness of the absorber, as well as the material composition of the absorber, is critical and determines the amount of attenuation and phase shift that occurs. For DUV wavelengths (193-248 nm), the attenuator thickness of Cr films becomes extremely small (&lt;100 .ANG.) and poses a significant practical problem in controlling its uniformity across a mask plate and then etching patterns reliably with control.
The phase shifting that occurs when light passes through the attenuator is dependent on but not necessarily a primary function of the thickness of the attenuator. Therefore, it is difficult to independently control the attenuation and phase shifting that occurs in the absorber material. Adjusting a parameter for one will cause the other to change as well. This type of PSM is called an imbedded PSM.
Another type of PSM uses a transparent dielectric layer of known thickness that is generally deposited above or below the absorber to provide for an additional phase shift to that of the absorber. This structure requires a second aligned lithography step and significantly complicates the overall manufacturing process. The precise alignment of the shifter layer to the absorber layer requires alignment marks and complicates the overall process. The PSMs using a deposited dielectric and absorber come in two varieties called top shifter and bottom shifter configurations.
Thus, there are three types of attenuated PSMs that are generally used in the prior art. The imbedded PSM requires precise control of the thickness for attenuation and phase shift, which can be achieved only over a limited range for materials such as chrome or chrome oxide. Other materials can be employed but the technology to deposit and etch them uniformly across the mask plate is very difficult and must be optimized for each material used. What is needed is a process where the attenuators can be fabricated without requiring an etch of the material. Also, one must be able to deposit the material to have a very smooth surface and have precise thicknesses.
Because of these limitations, prior art phase shifting masks that have provided adequate performance till now will have severe shortcomings as shorter wavelengths (for example, 193 nm) are exploited for conventional optical lithography. The present invention describes a phase shifting photomask having improved performance that is easily extendible to shorter wavelengths without adding additional complexity in manufacturing.